1. Field of the Invention
This invention relates to curable organosiloxane compositions. More particularly, this invention relates to one-part organosiloxane compositions that cure by means of a hydrosilylation reaction and contain a novel microencapsulated curing catalyst. The compositions exhibit long term storage stability under ambient conditions yet cure rapidly at elevated temperatures.
2. Description of the Prior Art
Organosiloxane compositions cure by a variety of reactions. Some of the more common curing means involve 1) a free radical reaction initiated by the heat-induced decomposition of an organic peroxide or the decomposition of a photoinitiator in the presence of ultraviolet light and 2) the reaction between a hydroxyl-containing polyorganosiloxane and an organosilicon compound containing two or more hydrolyzable groups that occurs under ambient conditions in the presence of moisture and a suitable catalyst.
One of the more useful classes of polyorganosiloxane compositions cures by a reaction between silicon-bonded hydrogen atoms and either silicon bonded alkenyl radicals or hydroxyl groups. These reactions are catalyzed by metals from the platinum group of the periodic table or compounds of these metals. The advantages of these compositions include their rapid curing rate, particularly at elevated temperatures, the absence of objectionable by-products produced during the curing of compositions containing organic peroxides or silanes with hydrolyzable groups such as acetoxy or methoxy, and the difficulty of achieving complete curing of moisture-curable organosiloxane compositions applied in thick layers.
Compositions that cure by a hydrosilylation reaction typically contain a polydiorganosiloxane with at least two ethylenically unsaturated hydrocarbon radical per molecule, an organohydrogensiloxane containing at least two silicon bonded hydrogen atoms per molecule in an amount sufficient to achieve curing of the composition and a platinum-or rhodium-containing catalyst in an amount sufficient to promote curing of the composition. Fillers and other additives may be present for the purpose of modifying physical and/or chemical properties of the composition either prior to or following curing.
Because organosiloxane compositions that cure by a platinum-catalyzed hydrosilylation reaction begin to cure even at ambient temperature once the reactants are combined, the catalyst and the organohydrogensiloxane reactant are usually packaged in separate containers and are not combined until it is desired to cure the composition. Even if the composition contains one or more of the known platinum catalyst inhibitors it cannot be stored in a single container for more than a few hours.
One of the alternatives proposed in the prior art to supplying platinum-catalyzed curable organosiloxane compositions as two-package materials is to isolate either the catalyst or the organohydrogensiloxane within a matrix of a material that is solid under the conditions encountered during storage of the curable composition, yet allows the entrapped reactant or catalyst to escape and mix with the other ingredients when it is desired to cure the composition.
The prior art discloses a number of different one-part curable organosiloxane compositions containing microencapsulated reactants or catalysts. An example of this type of composition is described in U.S. Pat. No. 4,528,354, which issued to McDougal and Dougherty on Jul. 9, 1985. This patent describes one-part peroxide curable silicone rubber compositions. The compositions include a microencapsulated liquid phase containing an organic peroxide in a shell of a resinous thermosetting material that is impervious to the peroxide.
The capsules are designed to rupture under a given internal vapor pressure that is generated by the encapsulated liquid when the curable composition containing the microcapsules is heated.
Because release of the peroxide is dependent upon rupturing rather than melting of the shell separating the peroxide from the other ingredients of the organosiloxane composition, the composition and thickness of the shell must be carefully controlled to ensure that the rupture of the capsules will occur reproducibly within the temperature range used to cure the organosiloxane composition.
U.S. Pat. No. 4,604,444, which issued to Donnadieu on Aug. 5, 1986 describes storage stable polyorganosiloxane compositions comprising a polyhydroxylated polyorganosiloxane, a polyfunctional acyloxysilane and a microencapsulated accelerator that either contains or generates water. The encapsulated material can be released using heat and/or irradiation. Suitable encapsulating materials include polystyrene, acrylonitrile-styrene copolymers, and poly(methyl methacrylate). This patent does not suggest using microencapsulated materials in organosiloxane compositions curable by means other than the reaction of polyhydroxylated polyorganosiloxanes with acyloxysilanes.
U.S. Pat. No. 4,461,854, which issued to Smith on Jul. 24, 1984 teaches two-part curable organosiloxane compositions. One part contains a silanol-terminated polyorganosiloxane and the second component contains a curing agent, a filler and an encapsulated catalyst. The catalyst is a specified group of metal salts of carboxylic acids where the metal is, for example, tin, lead or zirconium. The encapsulating material is preferably a salt of a carboxylic acid that does not promote room temperature curing of the composition. The encapsulated catalyst prolongs the bath life of the curable composition.
U.S. Pat. No. 4,481,341, which issued to Schlak et al. on Nov. 6, 1984 and Japanese published application No. 49/134,786, published on Dec. 25, 1974 describe thermosetting organosiloxane compositions comprising a polyorganosiloxane containing at least two ethylenically unsaturated hydrocarbon radicals per molecule, a polyorganohydrogensiloxane containing at least two silicon bonded hydrogen atoms per molecule and a platinum-containing catalyst that is dispersed in a finely divided, solid matrix, such as a silicone resin or an organic resin. The concentration of catalyst is from 0.001 to 5 percent by weight of platinum metal.
The finely divided material in which the catalyst is dispersed is virtually insoluble in either the aforementioned polyorganosiloxane or polyorganohydrogensiloxane and melts or softens at a temperature between 70 and 250 degrees C. The alleged advantage of the compositions disclosed in the patent to Schlak et al. is that the catalyst remains isolated from the other ingredients of the curable composition until the composition is heated sufficiently to melt the material in which the catalyst is dispersed. Because the organosilicon compounds present in the composition will not cure in the absence of the catalyst, the composition can allegedly be stored for long periods of time without undergoing curing or even an increase in viscosity.
A disadvantage of the curable organosiloxane compositions described by Schlak et al. and the published Japanese patent application is the method taught to prepare the catalyst/resin composition. A solid block or sheet of resin containing the platinum composition dispersed throughout is ground to a fine powder. Based on the random nature of the grinding operation there is a reasonable probability that some of the particles will contain platinum catalyst on their surface. Even trace amount of platinum have been shown to cause premature curing of the type of organosiloxane composition exemplified in this patent.
One way to avoid the inherent disadvantages of the catalyst compositions described in the Schlak et al. patent, is to completely microencapsulate finely divided particles or droplets of a catalyst composition within a material that is impermeable to the catalyst and effectively isolates it from the reactive ingredients of a curable organosiloxane composition. The encapsulant melts or softens at the desired curing temperature of the composition. A variety of methods for microencapsulating materials are known in the art.
U.S. Pat. No. 4,874,667, which issued on Oct. 17, 1989 to Lee et al. and is assigned to the same party as the present application discloses one-part organosiloxane compositions that cure by a platinum-catalyzed hydrosilylation reaction. The platinum catalyst is microencapsulated in one or two layers of a thermoplastic organic polymer. The diameter of the microencapsulated catalyst particles are less then 100 microns.
A disadvantage of the preparative methods described in the Lee et al. patent and related U.S. Pat. No. 4,766,176, issued on Aug. 23, 1988 and U.S. Pat. No. 4,784,879, issued on Nov. 15, 1988, is the inability of these methods to yield microcapsules of sufficiently small size that curable compositions containing these microcapsules are optically transparent. Curable compositions containing these microcapsules are translucent or opaque.
U.S. Pat. No. 5,066,699, which issued on Nov. 19, 1991 describes a method for preparing a microencapsulated platinum-containing hydrosilylation catalyst. A solubilized hydroxyl-containing ethylenically unsaturated organic compound is photopolymerized in the presence of the catalyst, a photoinitiator for the polymerization and an optional surfactant. Substantially none of the microcapsules is larger than 3 micrometers in diameter. This patent also discloses storage stable one-part curable organosiloxane compositions containing a microencapsulated catalyst prepared as described in the patent. The compositions are cured by heating them at a temperature that is at least equal to the softening temperature of the polymer portion of the microencapsulated catalyst.
One objective of this invention is to provide a novel type of microencapsulated platinum group metal-containing curing catalyst that does not detract from the optical transparency or the cure rate at elevated temperatures of curable organosiloxane compositions containing the catalyst.
A second objective is to provide a novel method for preparing microencapsulated hydrosilylation reaction catalysts that are effective curing catalysts for organosiloxane compositions.
Another objective is to provide optically transparent one-part storage stable organosiloxane compositions containing the microencapsulated curing catalysts of this invention. The compositions can be cured either by heating them or exposing them to ultraviolet radiation in the range of from 200 to 260 nanometers (nm).